In the manufacture of tissue and towel products, a common step is creping the product to provide desired aesthetic and performance properties to the product. Creping is commonly used in both the conventional wet press and through air drying processes. Many of the aesthetic properties of tissue and towel products rely more upon the perceptions of the consumer than on properties that can be measured quantitatively. Such things as softness, and perceived bulk are not easily quantified, but have significant impacts on consumer acceptance. However both softness and bulk are dramatically improved by the creping process. Creping is generally accomplished by mechanically foreshortening or compacting paper in the machine direction with a flexible blade, a so-called doctor blade, against a Yankee dryer in an on-machine operation. This blade is also sometimes referred to as a creping blade or simply a creper. By breaking a significant number of interfiber bonds and slowing down the speeds between the Yankee and the reel, creping increases the basis weight (mass per unit area) of the paper and effects significant changes in many physical properties, particularly when measured in the machine direction. Creping thus enhances bulk and stretch, and increases the perceived softness of the resulting product.
A Yankee dryer is a large diameter, generally 8-20 foot drum which is designed to be pressurized with steam to provide a hot surface for completing the drying of papermaking webs at the end of the papermaking process. The paper web which is first formed on a foraminiferous forming carrier, such as a Fourdrinier wire, where it is freed of the copious water needed to disperse the fibrous slurry, then is usually transferred to a felt or fabric either for dewatering in a press section where de-watering is continued by mechanically compacting the paper or by some other water removal method such as through-drying with hot air, before finally being transferred in the semi-dry condition to the surface of the Yankee for the drying to be completed. Before transferring to the Yankee dryer, an adhesive is applied directly to the Yankee dryer.
Obtaining and maintaining adhesion of tissue and towel products to Yankee dryers is an important factor in determining crepe quality. Re-wetability, doctorability, and the level of adhesion are important properties of a creping adhesive. The ability of the adhesive to be rewet on the surface of the dryer helps to prevent buildup on the drum and on the creping blade. Inadequate adhesion results in poor creping, sheet floating, and poor sheet handling whereas excessive adhesion may result in crepe blade picking, sheet plugging behind the crepe blade, and sheet breaks due to excessive tension. Traditionally, creping adhesives alone or in combination with release agents and/or modifiers have been applied to the surface of the dryer in order to provide the appropriate adhesion to produce the desired crepe. The adhesive coating also serves the purpose of protecting the Yankee dryer and creping blade surfaces from excessive wear. In this role, the coating agents provide improved runnability of the tissue machine. As creping blades wear, they must be replaced with new ones. This replacement process represents a significant source of tissue machine downtime, or lost production.
Various types of creping adhesives have been used to adhere fibrous webs to dryer surfaces such as Yankee dryers. Some examples of prior art creping adhesives rely upon combinations of self-crosslinkable soft polymers with a non-film forming hard polymer emulsion (U.S. Pat. No. 4,886,579). Some others involve thermoset resins (U.S. Pat. Nos. 4,528,316 and 4,501,640). The ability to control the mechanical properties of the polymers, as well as the adhesion and release of the fibrous web from the Yankee dryer, is limited when using these types of creping adhesives. A variety of proposals have been made in an attempt to improve the properties of certain adhesives. For example, U.S. Pat. No. 5,370,773 describes the use of a phosphate surfactant with an adhesive composition that includes a non-self-crosslinkable polymer or oligomer having functional groups that can be ionic crosslinked using a high valence metallic crosslinking agent. U.S. Pat. No. 6,280,571 describes the use of an acid selected from hypophosphorous acid, phosphorous acid, hypodiphosphoric acid, diphosphorous acid, hypophosphoric acid, pyrophosphorous acid, or their salts, to stabilize a polymer selected from polyamidoamine-epichlorohydrin resin, polyamine-epichlorohydrin resin, reaction products of epichlorohydrin with highly branched polyamidoamines and polyvinyl alcohol.
Poly(aminoamide)-epihalohydrin type creping adhesives (also referred to as PAE resins), exemplified by poly(aminoamide)-epichlorohydrin, provide a class of resins distinct from the above polymers. Resins of this type have been used for many years in paper making and are described in U.S. Pat. Nos. 2,926,116 and 3,058,873, the disclosure of which are incorporated herein by reference. They are generally prepared by reacting an epihalohydrin and a polyamide containing secondary or tertiary amine groups, followed by stabilizing the reaction products by acidification with sulfuric or hydrochloric acid. They have very useful properties when freshly applied in runnability and initial re-wetability and doctorability. However, a problem with the poly(aminoamide)-epihalohydrin type creping adhesives is the phenomenon of coating buildup. This problem is evidenced by high spots in the coating on the Yankee and/or build up on the rear surface of the blade, particularly along the edges or corners of the creping blade, which can cause chattering, or bouncing of the blade. Ultimately, portions of the sheet may travel underneath the creping blade, causing picks or holes in the sheet leading to sheet breaks and machine downtime. Commonly water sprays have been used to remove or minimize adhesive buildup, but eventually may prove inadequate.
In order to produce a bulky and soft tissue with conventional wet press paper machines, the paper sheet is preferably dried to very low moisture levels (e.g., less than 3%), thus economic considerations often require an adhesive that will perform at very high sheet temperatures. But the foregoing problems with the poly(aminoamide)-epihalohydrin type creping adhesives can be particularly severe at higher temperatures.
Another difficulty with PAE resins is the adverse effect of sizing agents such as alkyl ketene dimer (AKD), alkylene ketene dimers and alkylene succinic anhydride (ASA) on the creping process. These sizing agents, particularly AKD, are sometimes added to paper webs to impart moisture resistance properties for some special grades of paper. However, AKD performs as a strong release on the Yankee. When AKD is added to the furnish in the wet end, most of the PAE adhesives have issues in generating sufficient adhesion between the Yankee surface and the sheet often resulting in poor creping and sheet handling issues or limiting the amount of these sizing agents that can be incorporated into the sheet if good creping is desired.